download gapps here : http://opengapps.org/
i used ARM 7.1 Pico. its the smalest version of gapps, it only install play store.
Put the .zip file you just download on your usb stick.

second :
start the Pi3 with Emteria.OS, when it's start plug your usb.
open the usb via the upper side of you screen.
press a long time on the gapps.zip file and copy past to Download folder.
eject your usb.

third :
Use the Shutdown preinstalled app and go to RECOVERY.
in the recovery menu , use "install" an go to the download folder. select your gapps.zip file.
and clic on install from zip.
select reboot after install.
wait and